1. Field of the Invention
The present invention relates to a method of forming a structured fiber web on a paper machine, and, more particularly, to a method and apparatus of forming a structured fiber web on a structured forming fabric in a paper machine.
2. Description of the Related Art
In a wet molding process, a structured fabric in a Crescent Former configuration impresses a three dimensional surface on a web while the fibrous web is still wet. Such an invention is disclosed in International Publication No. WO 03/062528 A1. A suction box is disclosed for the purpose of shaping the fibrous web while wet to generate the three dimensional structure by removing air through the structural fabric. It is a physical displacement of portions of the fibrous web that leads to the three dimensional surface. Similar to the aforementioned method, a through air drying (TAD) technique is disclosed in U.S. Pat. No. 4,191,609. The TAD technique discloses how an already formed web is transferred and molded into an impression fabric. The transformation takes place on a web having a sheet solids level greater than 15%. This results in a low density pillow area in the fibrous web. These pillow areas are of a low basis weight since the already formed web is expanded to fill the valleys thereof. The impression of the fibrous web into a pattern, on an impression fabric, is carried out by passing a vacuum through the impression fabric to mold the fibrous web.
It is known to form a fiber web in a wet molding process using a structured fabric to impress a three dimensional surface on the web while the fibrous web is still wet. Such an invention is disclosed in International Publication No. WO 03/062528 A1. It is known to use forming fabrics, which have a load bearing layer and a sculptured layer wherein impression knuckles are formed, which imprint the sheet to increase the surface contour. Such an invention is disclosed in U.S. Pat. No. 5,429,686. However, this patent does not teach the creation of pillows on a sheet that are required for effective dewatering in through air drying (TAD) applications and in particular of an ATMOS™ papermaking machine. U.S. Pat. No. 6,237,644 teaches the use of fabrics, which are woven with a lattice pattern of at least three yarns oriented in both warp and weft. This reference teaches the use of a pattern fabric to provide shallow craters in distinct patterns. The physical displacement of portions of the fibrous web is a technique utilized to lead to a three-dimensional surface. A TAD technique is disclosed in U.S. Pat. No. 4,191,609. The TAD technique discloses how an already formed web is transferred and molded into an impression fabric. The transformation takes place on a web having a sheet solids level greater than 15%. This results in a low density pillow area in the fibrous web having a low basis weight, since the already formed web is expanded to fill the valleys. The impressions of the fibrous web into a pattern is carried out by passing a vacuum through the impression fabric to mold the fibrous web.
Prior art weave patterns such as the M weave illustrated in FIGS. 19-21 and the G weave shown in FIGS. 22-24 illustrate prior art fabrics that limit the amount of bulk that can be built into the fibrous web due to the shallow depth of the pockets. The weave patterns of the M weave and G weave are each based on a 5 by 5 pattern, which serves to define the location and shape of pockets. The pockets in these fabrics are shown as the darkened areas in FIGS. 19 and 22. These pockets are of such shape and depth that the bulk that can go therein is limited to less than a desired amount.
What is needed in the art is a structured forming fabric that will provide increased caliper, bulk and absorbency in tissue and toweling formed thereon.